This invention relates to a combination expulsion fuse, that is it relates to a fuse having a current limiting section and an expulsion section.
Current limiting fuses and low current expulsion fuses are well known in the art. A current limiting fuse is used where there are high fault currents in order to limit the high current when a fault occurs, and to restrict the current to a lower magnitude that can be handled by another fuse or trip arrangement. An expulsion fuse is for lower fault currents and it normally has an open ended expulsion tube which contains a fuse link with a flexible cable secured to it. The cable extends out the open end of the tube and is fastened to a spring arrangement which places a force on the cable keeping it in tension. The expulsion tube is frequently coated on its inner surface with a composition which releases moisture when an arc is formed within the tube thereby cooling the arc, absorbing some arc energy which helps to extinguish the arc. Melting of the fuse link under fault current causes an arc within the tube which generates gases. The gases and the spring force on the cable cause the fuse link cable to be expelled or ejected, and the expelled cable produces a visual air gap indicating a blown fuse.
In recent years the use of capacitor banks for phase angle correction in electrical systems has become commonplace. The banks are made up of a number of capacitors in a series/parallel arrangement to provide the required correction at the necessary voltages and it is very desirable to have a fuse in series with each capacitor. If a capacitor fails all those in parallel with it will tend to discharge through the failed capacitor and this could involve a considerable amount of electrical energy. If the failed capacitor is not removed from the circuit quickly the large amounts of energy could rupture the capacitor and could damage the bank. It was found that a desirable protective means is to use, in series with each capacitor, a combination fuse which has a current limiting section and an expulsion section in series. If a capacitor fails the large fault current will be limited by the current limiting portion and this limited current will cause the fuse link in the expulsion section to blow with a resulting indication of a blown fuse and a failed capacitor.
The combination fuse, that is the combination current limiting fuse and expulsion fuse, may be used for other purposes than the protection of capacitors but they are particularly suitable for use with capacitors and will be described in that respect.
Known designs of combination fuses have the current limiting section and the expulsion section in electrical series relationship and in mechanical series relationship. The current limiting section has a long cylindrical casing with a closure cap of conducting metal at each end. The caps may serve as contact or terminal members. Within the casing is a centrally located rod of insulating material. This rod may support the fuse wires or fuse elements or it may be surrounded by a spiral fuse element for which it does not provide direct support. The rod may have spiral grooves on the surface to hold fuse wires or fuse elements. The fuse wires, of silver or other suitable metal, extend from one closure cap to the other. The casing is filled with pulverulent arc quenching material such as silicon dioxide sand. Under fault conditions the fuse wires melt and/or vapourize and the fused metal condenses in the sand. Any arcs formed are confined and cooled by the sand. The restriction of the arc produces a high arc voltage which tends to oppose the system voltage and limits the current.
Normally one of the closure caps is mounted to a bus bar in the capacitor bank and the fuse is supported from this mounting. Attached to the closure cap at the other end is the expulsion tube of the low current expulsion section so that the current limiting and expulsion sections are in end to end abutting relationship (i.e., a mechanical series arrangement). One end of a fuse link is attached to the closure cap and a flexible fuse link cable extends from the other end of the link down the expulsion tube and out the open end of the tube to connect with a terminal of a capacitor. A spring arm or other spring means places tension on the cable. As was previously explained, melting and/or vapourization of the fuse link in the expulsion tube creates an arc which forms gases and these gases tend to expel the cable. This is assisted by the spring tension on the cable.
It is desirable to have a relatively long expulsion tube as the tube tends to cool the arc and extinguish it. The longer the tube, the greater the cooling action. However, in the prior art the length of the expulsion tube is necessarily limited because the combination fuse is supported or mounted at one end and there is a practical limit to the length the fuse can extend from this support. The fuses are normally mounted at an angle, inclined downwardly from the supporting end, to reduce the amount of moisture and dirt which might otherwise enter the expulsion tube. A long fuse increases the strain on the mounting and increases the tendency for vibration.
The present invention provides for a structural arrangement permitting a longer expulsion section with a shorter overall length to the combination fuse. In addition, the arrangement provides for a mounting terminal adjacent the end of the current limiting section from which the expulsion tube extends thereby reducing the distance the fuse extends from the mounting point to reduce the strain on the mounting and reducing the tendency to vibrate.